The objective of audio encoding is compression encoding a digitalized signal as effectively as possible, transmitting the encoded signal, and reproducing an audio signal of the highest possible quality through decoding of the encoded signal by a decoder.
Various methods have been proposed as audio encoding methods, depending on the conditions such as the type of the signal to be encoded, the bit rate, and required sound quality. For example, MPEG-4 Audio which is an ISO/IEC standard specification (see Non-patent Reference 1) discloses encoding methods such as Advanced Audio Coding (AAC), Code Excited Linear Prediction (CELP), and HVXC (Harmonic Vector eXcitation Coding). In particular, the AAC method is an excellent method that can encode, with high quality (on par with compact disc audio, for example), a general audio signal that contains music, and is characterized in utilizing a time-frequency transformation called Modified Discrete Cosine Transform (MDCT). These encoding methods are widely used in communication, broadcasting, and accumulation-type audio devices.
On the other hand, in the listening/viewing of broadcast or accumulated audio or audio/video composite information, there is an increasing demand for making reproduction speed during listening/viewing variable. With the increased capacity of information accumulation means and diversification of information obtainment methods, the amount of information that can be viewed/listened to by an individual has increased dramatically. Therefore, a high-speed reproduction function for viewing/listening to more information within a limited time is important.
As a method for variable-speed reproduction of an audio signal, there is a first method which cancels and inserts a pitch waveform, based on the pitch cycle of a temporal audio signal (see Patent Reference 1), and a second method which, after the parameter transformation of an audio signal, changes the update cycle of the parameters (see Patent Reference 2). However, as a processing method for a high-quality input signal, the use of the pitch cycle-based temporal signal processing in the former is common. This is because the second method is only used in low-quality speech, and is not suitable for a high-quality signal.
An example of the configuration of an audio decoding apparatus for realizing variable-speed reproduction of an audio signal encoded using an MDCT-based audio encoding method is shown in FIG. 1.
As shown in FIG. 1, a decoding apparatus 9000 includes a bitstream separation unit 9901, an MDCT coefficient decoding unit 9902, an inverse MDCT unit 9903, a pitch analyzing unit 9904, a reproduction speed control unit 9905, a waveform modification unit 9906, and a waveform connecting unit 9907.
An input bitstream 9908 is separated into respective code elements by the bitstream separation unit 9901. An MDCT code 9909, which is a code element required in decoding an MDCT coefficient, is inputted to the MDCT coefficient decoding unit 9902, and an MDCT coefficient 9910 is decoded. The inverse MDCT unit 9903 performs inverse-transformation on the MDCT coefficient 9910, and a temporal audio signal 9911 is generated. The pitch analyzing unit 9904 analyzes the pitch cycle of the temporal audio signal 9911. The reproduction speed control unit 9905, upon receiving a reproduction speed change instruction 9913, determines a start position 9914 for reproduction speed changing based on analyzed pitch cycle 9912. The waveform modification unit 9906 performs the modification of the waveform (waveform cancellation and insertion) based on the pitch cycle 9912 at the start position 9914 for the processing, connects the modified waveform 9915, and generates an output audio signal 9916.
Furthermore, as shown (in Patent Reference 3), it is also possible to have a configuration which makes use of pitch cycle information included in the input bitstream, instead of the pitch cycle 9912 analyzed by the pitch analyzing unit 9904.    Patent Reference 1: Japanese Patent No. 3147562    Patent Reference 2: Japanese Unexamined Patent Application Publication No. 9-6397    Patent Reference 3: PCT International Patent Application Publication No. 98/21710 (Pamphlet)    Non-patent Reference 1: ISO/IEC 14496-3:2001    Non-patent Reference 2: IEEE Trans. ASSP-34 No. 5, October 1986, John P. Princen and Alan Bernard Bradley, “Analysis/Synthesis Filter Bank Design Based on Time Domain Aliasing Cancellation”